The invention is used to control dose enhancements along a photon beam path by control of the magnetic field configuration of a topical magnet, the magnetic field configuration having a magnetic field component across the photon beam path and having a magnetic field gradient component along the photon beam path.
Since the advent of radiation systems workers have long been seeking methods and devices to control dose enhancements, where a dose enhancement is the ratio of radiation dose in a target volume relative to the radiation dose outside of the target volume. For example, one of the fundamental problems in the treatment of many forms of cancer using beams of high energy photons (mainly in the range 1 MEV to 60 MEV) from accelerators and other sources is the limited success of current techniques for delivering appropriate levels of dose to a diseased region while sparing surrounding healthy tissue.
The dose generated by a high energy photon beam comes from the loss of energy of Compton and pair production electrons in an electron-photon cascade generated by the photon beam. (The differences in charge and particle interactions between electrons and positrons are minimal in the phenomena relied on here, so positrons created by pair production are called simply electrons here.) The electron-photon cascade follows the photon beam progression, and scattering into the penumbral region around the beam is usually acceptably small. Thus, healthy regions lying in directions transverse to the beam direction can usually be protected by shaping the photon beam cross-section by means of absorber blocks and related techniques. In addition, the targeted region, when possible, is irradiated from various directions so as to spare any particular region of surrounding healthy or especially radiosensitive tissue from the full destructive impact of the treatment. For a photon beam incident from any given direction, however, there heretofore has been no effective means of minimizing damage to healthy tissue not in the target volume. No suggestions have been made that dose enhancements along uncharged photon beams could be controlled by control of the magnetic field configuration of a topical magnet, the magnetic field configuration having a magnetic field component across the photon beam path and having a magnetic field gradient component along the photon beam path.
Suggestions for improving the dose distribution along a charged particle beam by use of magnetic fields have been made. In C. C. Shih, "High Energy Electron Radiotherapy in a Magnetic Field," Medical Physics, Vol. 2, No. 1, January/February 1975 calculations are reported which suggest that an electron beam dose distribution could be improved in the uniform magnetic field of a large magnet. In Whitmire, D. P., Bernard, D. L., Peterson, MD, and Purdy, J. A., "Magnetic Enhancement of Electron Dose Distribution in a Phantom," Medical Physics, Vol. 4, No. 2, March/April 1977 measurements of dose in a phantom in the uniform magnetic field of a large magnet are reported which also suggest that an improved dose distribution could be achieved by these means.
Similar work is reported in Nath, R. and Schulz, R. J., "Modification of Electron-beam Dose Distributions by Transverse Magnetic Fields," Medical Physics, Vol. 5, No. 3, May/June 1978; in Whitmire, D. P. Bernard, D. L. and Peterson, M.D., "Magnetic Modification of the Electron-Dose Distribution in Tissue and Lung Phantoms," Medical Physics, Vol. 5, No. 5, September/October 1978; in Paliwal, B. R., Wiley, Jr., A. L., Wessels, B. W. and Choi, M. C., "Magnetic Field Modification of Electron-beam Dose Distributions in Inhomogeneous Media," Medical Physics, Vol 5, No. 5 September/October 1978; and in Paliwal, B. R., Thomadsen, B. R. and Wiley, Jr., A. J., "Magnetic Modification of Electron Beam Dose Distributions," Acta Radiological Oncology, Vol. 18, 1979 Fasc. 1.
None of these workers suggest that dose enhancements along a photon beam could be controlled by controlling the configuration of a topical magnet magnetic field having a magnetic field component across the photon beam and having a magnetic field gradient component along the photon beam. The 1978 Whitmire paper mentions an increase in dose from a photon beam at the surface of a phantom in their magnetic field and a decrease in dose at the bottom of their phantom. Their discussion of this observation teaches away from control of dose enhancements by control of the configuration of the magnetic field of a topical magnet.
In Weinhous, M. S., Nath, R. and Schuylz, R. J., "Enhancement of Electron Beam Dose Distributions by Longitudinal Magnetic Fields: Monte Carlo Simulations and Magnet System Optimization," Medical Physics, Vol. 12, No. 5 September/October 1985 and in Bielajew, A. F., "The Effect of Strong Longitudinal Magnetic Fields on Dose Deposition from Electron and Photon Beams," Medical Physics, Vol. 20, No. 4, July August 1993 calculations are reported to suggest that large uniform magnetic fields along the beam axis would reduce the scattering of electrons laterally out of the beam. In the case of the photon beam, the electrons in the electron-photon cascade which are scattered transverse to the beam are kept in the beam, thereby somewhat reducing the dose in the penumbral region around the beam. Their discussion of this effect teaches away from using a topical magnet with a gradients along a photon beam path.